Cation deficient materials for electrical energy storage

ABSTRACT

A composition comprising: a metal oxide of a first metal ions and second metal ions; an electrically conductive material; and a binder material. The second metal ions have a higher oxidation state than the first metal ions. The presence of the second metal ion increases the number of metal cation vacancies. A method of: dissolving salts of a first metal ion and a second metal ion in water to form a solution; heating the solution to a temperature of about 80-90° C.; and adding a base to the solution to precipitate nanoparticles of a metal oxide of the first metal ion and the second metal ion.

This application is a divisional application of U.S. patent applicationSer. No. 12/855,144, allowed and filed on Aug. 12, 2010, which claimsthe benefit of U.S. Provisional Application No. 61/233,948, filed onAug. 14, 2009. The provisional application and all other publicationsand patent documents referred to throughout this nonprovisionalapplication are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is generally related to materials for lithium-ionbatteries.

DESCRIPTION OF RELATED ART

The demand for lithium-ion batteries has increased rapidly over the pasttwo decades, and as technology continues to advance, manufacturers willwant to develop low-cost, nontoxic cathode materials with exceptionalcharge-storage properties. Iron oxides are naturally abundant andenvironmentally safe, but the reported Li⁺ capacities are often lowerthan metal oxides used in commercial electrode materials, such as LiCoO₂(Manthiram et al., Chem. Mater. 10, 2895-2909 (1998)). Additionally, theinsertion potential for Li⁺ is limited by the Fe^(2+/3+) redox couple,and as a result, many iron oxides insert Li⁺ at potentials so low(usually <2 V vs Li) (Reddy et al., Adv. Funct. Mater. 17, 2792-2799(2007); Liu et al., Electrochim. Acta 54, 1733-1736 (2009); Abraham etal., J. Electrochem. Soc. 137, 743-749 (1990); Prosini et al., Int. J.Inorg. Mater. 2, 365-370 (2000); Lipparoni et al., Ionics 8, 177-182(2002)), they are not practical as cathode materials for lithium-ionbatteries.

Despite these limitations, the electrochemical properties of iron oxidecan be significantly enhanced by tailoring the particle size andstructure of the active material. For example, reducing the particlesize increases the electrochemically active surface area per structuralunit and allows iron oxide to be more accessible to the electrolyte. Asa result, the average distance electrons and charge-compensating cationsmust travel to reach the intercalation host is reduced and higher Li⁺storage capacities can be achieved (Bazito et al., J. Braz. Chem. Soc.17, 627-642 (2006); Aricò et al., Nature Mater. 4, 366-377 (2005)). Thisprinciple has been experimentally demonstrated with hematite, α-Fe₂O₃,where large capacities of 250 mAh g⁻¹ were attained when the particlesize was reduced to ˜5 nm (Jain et al., Chem. Mater. 18, 423-434(2006)). Other advantages realized by expressing iron oxide at thenanoscale include faster rate capabilities and greater long-termstability during electrochemical cycling (Hibino et al., J. Electrochem.Soc. 154, A1107-A1111 (2007)), because smaller particles can more easilyaccommodate the strain induced during charge storage.

The oxide structure is also related to the mechanism for solid-stateinsertion of small cations and the accompanying phase transformationsthat occur during electrochemical cycling, which determine how much Li⁺can be reversibly stored in the host material. For example, the spinelferrite magnetite, Fe₃O₄, cannot structurally accommodate reversiblecharge storage, because the injection of Li⁺ into the latticeirreversibly displaces tetrahedrally coordinated Fe³⁺ into octahedralsites (Thackeray et al., Mat. Res. Bull. 17, 785-793 (1982); ThackerayJ. Am. Ceram. Soc. 82, 3347-3354 (1999)). Maghemite, γ-Fe₂O₃ orFe[Fe_(5/3).□_(1/3)]O₄, is an isostructural analog of magnetitecontaining cation vacancies (□) in the octahedral Fe²⁺ positions. Thesevacancies increase the Li⁺ storage capacity and shift the electromotiveforce (EMF) of electrochemical charge storage to more positivepotentials (Pernet et al., Solid State Ionics 66, 259-265 (1993)).Similar observations correlating the cation vacancy content of an oxidewith enhancements to the classical small cation insertion mechanism havebeen made using defect MnO₂ phases (Ruetschi, J. Electrochem. Soc. 1352657-2663 (1988); Ruetschi et al., J. Electrochem. Soc. 135, 2663-2669(1988)) and microcrystalline V₂O₅ (Swider-Lyons et al., Solid StateIonics 152-153, 99-104 (2002)).

Gillot and coworkers demonstrated that substituted spinel ferritescontaining highly oxidized cations (e.g., Mo⁶⁺, V⁵⁺) can be preparedusing a chimie douce approach (Gillot et al., Solid State Ionics101-103, 261-264 (1997); Gillot et al., Heterogen. Chem. Rev. 1, 69-98(1994)). The resultant solids are cation-deficient with a vacancypopulation up to 2-3 times that of γ-Fe₂O₃ (Gillot et al., Mat. Res.Bull. 34, 1735-1747 (1999); Gillot et al., Solid State Ionics 52,285-286 (1993)).

BRIEF SUMMARY

Disclosed herein is a composition comprising: a metal oxide comprisingions of a first metal and ions of a second metal; an electricallyconductive material; and a binder material. The second metal ions have ahigher oxidation state than the first metal ions. The presence of thesecond metal ion increases the number of metal cation vacancies.

Also disclosed herein is a composition comprising a metal oxidecomprising ions of a first metal and ions of a second metal. The secondmetal ions have a higher oxidation state than the first metal ions. Thepresence of the second metal ion increases the number of metal cationvacancies. The metal oxide is proton stabilized.

Also disclosed herein is a method comprising: dissolving salts of afirst metal ion and a second metal ion in water to form a solution;heating the solution to a temperature of about 80-90° C.; and adding abase to the solution to precipitate nanoparticles of a metal oxide ofthe first metal ion and the second metal ion. The second metal ions havea higher oxidation state than the first metal ions. The presence of thesecond metal ion increases the number of metal cation vacancies.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIG. 1 shows XRD patterns of “as-prepared” Mo-ferrite and Fe_(3-x)O₄depicted against known diffraction patterns of Fe₃O₄ (PDF #89-0688) andγ-Fe₂O₃ (PDF #39-1346).

FIG. 2 shows high resolution XPS spectra of the Mo 3d region taken ofMo-ferrite. The background subtracted data (solid line) and fit (dashedline) are shown. χ² _(red)=1.446.

FIG. 3 shows representative TEM images of (a) Fe_(3-x)O₄ and (b)Mo-ferrite displayed with inset micrographs showing the correspondingelectron diffraction patterns. The numeric labels identify the hklindices.

FIG. 4 shows bright field and scanning transmission electron microscopy(STEM) images of Mo-ferrite shown with EDX maps of the Mo, Fe, and Ocontent.

FIG. 5 shows TG-MS data of (a) Mo-ferrite and (b) Fe_(3-x)O₄ where thedashed line indicates the change in mass and the solid line indicatesthe ion current detected by the mass spectrometer at m/z=18.

FIG. 6 shows cyclic voltammograms of (a) Fe_(3-x)O₄ and (b) Mo-ferritecomposite electrodes immersed in 1 M LiClO₄/propylene carbonate.Electrodes were poised at 4.1 V vs Li for 10 min and then scanned at 500μV s⁻¹ to 3.0 V, 2.5 V, and 2.0 V vs Li as shown. Current is normalizedto the metal oxide mass.

FIG. 7 shows galvanostatic charge-discharge curves of (a) Fe_(3-x)O₄ and(b) Mo-ferrite composite electrodes immersed in 1 M LiClO₄/propylenecarbonate. Cycles 1-3 are shown. The applied current was 10 mA g⁻¹.

FIG. 8 shows galvanostatic charge-discharge curves of a Mo-ferritecomposite electrode immersed in 1 M Mg(ClO₄)₂/propylene carbonate.Cycles 1-3 are shown. The applied current was 20 mA g⁻¹.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

Disclosed herein are defect iron oxides, such as nanocrystallineγ-Fe₂O₃, modified to serve as enhanced cathode materials for lithium-ionbatteries. Given that there is a strong relationship between vacancycontent and energy-storage capabilities, optimized Mo- and V-substitutediron oxide spinels may achieve reversible Li⁺ capacities in excess of100 mAh g⁻¹ at a discharge potential of 2 V vs Li. Moreover, theinclusion of Mo⁶⁺ and V⁵⁺ as substituent cations is anticipated to shiftthe EMF of Li⁺ insertion to approximately 2.7 V and 3.4 V vs Lirespectively, consistent with previous charge-storage studies involvingthe Mo^(6+/5+) and the V^(5+/4+) redox couples (Ohtsuka et al., SolidState Ionics 144, 59-64 (2001); Guzman et al., Solid State Ionics,86-88, 407-413 (1996); Vivier et al., Electrochim. Acta 44, 831-839(1998)).

γ-Fe₂O₃ is a potentially better cathode material than Fe₃O₄, because theoxide structure has Fe²⁺ vacancies that insert Li⁺ prior to structuraltransformations (Pernet et al., Solid State Ionics 66, 259-265 (1993)).As an extension of this concept, more highly cation-deficient spinelsshould perform as better Li⁺ intercalation hosts than γ-Fe₂O₃. In caseswhere the defect spinel is a binary oxide, such as Mo_(a)Fe_(b)O₄ andV_(a)Fe_(b)O₄, the choice of substituent may enhance the charge-storagemechanism further by raising the Li⁺ insertion potential. The materialmay also be a magnesium insertion material. Herein is described thesynthesis and characterization of the substituted spinel ferrite, Mo⁶⁺_(0.59)Fe³⁺ _(1.45)Fe²⁺ _(0.06)□_(0.90)O₄.nH₂O (hereby designated asMo-ferrite), and the electrochemical charge-storage mechanism withrespect to the parent oxide, Fe³⁺ _(2.36)Fe²⁺ _(0.46)□_(0.18)O₄.nH₂O(hereby designated as Fe_(3-x)O₄), where □ represents the vacancycontent.

γ-Fe₂O₃ and other oxides may be improved as an intercalation host byincreasing the number of cation vacancies within the defect spinelframework. Published studies by Gillot and coworkers proposed (Gillot etal., Heterogen. Chem. Rev., 1, 69-98 (1994); Nivoix et al., Chem.Mater., 12, 2971-2976 (2000)) and then demonstrated (Gillot, Vib.Spectrosc., 6, 127-148 (1994); Domenichini et al., Thermochim. Acta,205, 259-269 (1992); Gillot et al., Mat. Res. Bull., 34, 1735-1747(1999)) that a fraction of the Fe³⁺ sites within Fe₃O₄ can be replacedwith more highly oxidizable cations, such as Mo³⁺ or V³⁺, whilepreserving the inverse spinel structure. When these transition metalcations undergo multi-electron oxidation processes during sintering,defect spinels form with the Mo- and V-substituted ferrites achievingover twice as many vacancies per structural unit as γ-Fe₂O₃ (Table 1).This hypothesis has been tested experimentally. Considering these highlycation-deficient frameworks, it is predicted that Mo_(a)Fe_(b)O₄ andV_(a)Fe_(b)O₄ spinels are better Li⁺ intercalation hosts than γ-Fe₂O₃.Not only are these binary metal oxides structurally designed toaccommodate higher Li⁺ storage capacities, but in addition, theinclusion of Mo⁶⁺ and V⁵⁺ as dopants is anticipated to shift the EMF ofLi⁺ intercalation to more positive potentials. The pure oxides Mo⁶⁺O₃and V⁵⁺ ₂O₅ begin to insert Li⁺ around 2.7 V and 3.4 V, vs Lirespectively (Ohtsuka et al., Solid State Ionics, 144, 59-64 (2001);Guzman et al., Solid State Ionics, 86-88, 407-413 (1996); Vivier et al.,Electrochim. Acta, 44, 831-839 (1998)), and it is expected that anymulti-electron reductions that occur during discharge (e.g.,Mo⁶⁺→Mo⁵⁺→Mo⁴⁺) will support the enhanced Li⁺ capacities that resultfrom high vacancy concentrations. Other example second metals include,but are not limited to, Nb, Ta, Ti, W, and Zr. The first metal and thesecond metal may be different and they may both be other than Li.

TABLE 1 Theoretical vacancy concentrations of Mo— and V— substitutedspinel ferrites Vacancies per # of Cation Defect Spinel^(a) Formula UnitVacant Sites^(b) Fe³⁺ _(8/3)•□_(1/3)O₄ 0.33 2.67 Mo⁶⁺ _(2/3)•Fe³⁺_(4/3)•□_(3/3)O₄ 1.00 8.00 V⁵⁺ _(8/11)•Fe³⁺ _(16/11)•□_(9/11)O₄ 0.826.63 ^(a)Formulas are described in AB₂O₄ stoichiometry. ^(b)Number ofvacancies per unit cell. Assumes 24 cation sites possible.

At low temperatures (˜250-400° C.) Fe₃O₄ undergoes a topotacticrearrangement to form maghemite γ-Fe₂O₃ (Cornell et al., The IronOxides: Structure, Properties, Reactions, Occurrence, and Uses. VCH:Weinheim, 1996; Zboril et al., Chem. Mater., 14, 969-982 (2002)), acation-deficient analogue that is more electrochemically stable atpositive potentials. Structurally magnetite and maghemite are almostidentical, except maghemite has vacancies in the octahedral Fe²⁺positions. (Although this is generally true, it is possible to preparehighly disordered γ-Fe₂O₃ phases containing both tetrahedral andoctahedral vacancies. For further discussion, see Humihiko et al., J.Phys. Soc. Jpn., 21, 1255-1263 (1966).) Pernet and coworkers (Pernet etal., Solid State Ionics, 66, 259-265 (1993)) suggest that these vacantsites electrochemically intercalate Li⁺ prior to major structuraltransformations, resulting in a more positive EMF for Li⁺ insertion andhigher charge storage capacities. Furthermore, when prepared innanocrystalline form, γ-Fe₂O₃ has been shown to insert/extract Li⁺ atCoulombic efficiencies up to 98% with little capacity fading after thefifth charge-discharge cycle (Quintin et al., Electrochim. Acta, 51,6426-6434 (1993)).

Using a chimie douce approach, Mo-ferrite or other substituted defectspinel ferrites may be prepared through a base-catalyzed precipitationreaction. In our studies, the material composition was establishedthrough X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray(EDX) spectroscopy, X-ray absorption spectroscopy (data not shown), andthermogravimetry/mass spectrometry (TG-MS). Structural and morphologicalproperties were established using X-ray diffraction (XRD) andtransmission electron microscopy (TEM). Finally, the electrochemicalcharge storage mechanism was examined through cyclic voltammetry andgalvanostatic charge-discharge experiments in a conventionalthree-electrode half-cell configuration.

Substitutionally doping Mo⁶⁺ for Fe³⁺ in γ-Fe₂O₃ is proposed to increasethe relative population of cation vacancies within the lattice. Forevery cation vacancy created, charge neutrality must be maintained byeither losing a proportional amount of anion charge or accepting otherpositively charged species (e.g., H⁺, Mo⁶⁺).

The extent to which water (or protons) participate to stabilizecation-deficient lattice structures is not well understood, but thelocal coordination geometry and the mechanism of defect formation areboth strongly influenced by the synthetic pathway. Considering that ourferrite synthesis is an aqueous chimie douce route, coordinated water islikely incorporated into our material composition during the formationof transition metal cation vacancies. Theoretical models addressing thistype of defect lattice structure have been described forcation-deficient phases of MnO₂, where four protons are proposed tooccupy each vacant Mn⁴⁺ site to preserve charge neutrality (Ruetschi etal., Electrochem. Soc., 135, 2663-2669 (1988)). The existence of theseprotons is justified experimentally by thermogravimetric studies. Forexample, when cation-deficient MnO₂ is heated up to 400° C., thevacancies and charge compensating protons become mobile; as vacanciesare removed, the trapped protons are released to react with O²⁻ and formwater (Ruetschi et al., Electrochem. Soc., 135, 2663-2669 (1988)). If asimilar charge-compensation mechanism applies to Mo-ferrite, watershould be expelled from the sample at temperatures significantly beyond100° C. The proton-stabilized form of our synthesized maghemite and Mosubstituted forms may be expressed as Fe_(2.36) ³⁺Fe_(0.46)²⁺□_(0.18)O₄.nH₂O and Mo⁶⁺ _(0.59)Fe³⁺ _(1.45)Fe²⁺_(0.06)□_(0.90)O₄.nH₂O, respectively. Any reference to an oxidethroughout this application can include such a proton-stabilized formwithout specifically referencing the protons or hydrates.

Assuming the inclusion of protons/water within the defect ferrites isrequired to maintain structural stability, one can write down ahypothetical equilibrium between the hydrated, cation-deficient γ-3phaseand the dehydrated, defect-free solid product(s) to solve for the watercontent (Eqs. 1, 2).

Fe_(2.36) ³⁺Fe_(0.46) ²⁺□_(0.18)O₄.nH₂O_((s))+0.115 O_(2(g))→1.41Fe₂O_(3(s))+nH₂O_((g))  (Eq. 1)

Mo⁶⁺ _(0.59)Fe³⁺ _(1.45)Fe²⁺ _(0.06)□_(0.90)O₄.nH₂O_((s))+0.0175O_(2(g))→0.558 3 Fe₂O_(3(s ))+0.196 7 Fe₂(MoO₄)_(3(s))+nH₂O_((g))  (Eq.2)

The utilization of Mo- and V-substituted spinel ferrites as cathodematerials in lithium-ion batteries may be highly economical compared tothe cost of developing more traditional electrode materials (e.g.,LiCoO₂). Iron-based compounds can be 60-160 times less expensive thanmany commercially developed metal oxides used for energy storage (Choiet al., J. Power Sources 163, 158-165 (2006)), so there is tremendousincentive to rethink/redesign intercalation hosts using iron. Prior tothis work, iron oxides were less practical as cathode materials, becausegenerally only small capacities could be achieved at a dischargepotential limit of 2 V vs. Li. The mixed metal oxide nanoparticlesreported herein can exhibit reversible Li⁺ capacities several timeshigher than those observed in classical γ-Fe₂O₃ and express a morepositive EMF for Li⁺ intercalation. The combination of enhancements inboth capacity and EMF in Mo- and V-substituted spinel ferrites isanticipated to increase the energy density of Li-ion batteries utilizingsaid cathode materials by an order of magnitude or more.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

EXAMPLE 1 Synthesis of Fe_(3-x)O₄ and Mo-ferrite Nanoparticles

The synthesis for developing cation-deficient spinel ferrites wasadapted from a chimie douce approach described by Gillot and coworkers(Gillot et al. Solid State Ionics, 52, 285-286 (1992); Gillot et al.,Solid State Ionics, 63-65, 620-627 (1993)), but the literature suggestsother aqueous (Livage, J. Sol-Gel Sci. Technol., 1, 21-33 (1993);Herranz et al., Chem. Mater., 18, 2364-2375 (2006)) and organic (Gash etal., Chem. Mater., 13, 999-1007 (2001)) routes could be modified toprepare similar materials. For this study, Mo-ferrite nanoparticles wereprepared by dissolving 20 mmol of MoCl₅ (Strem), FeCl₃.6H₂O (Aldrich),and FeCl₂.4H₂O (Aldrich) in 350 mL of nanopure water (“nH₂O”). (MoCl₅ ishighly reactive in the presence of water and liberates toxic HCl gas.Take appropriate safety precautions.) While stirring, the temperature ofthe solution was raised to 80-90° C., and triethylamine (Aldrich) wasadded dropwise in a 10:1 organic-to-metal mole ratio. An opaque solidwas soon visible, and after one hour of reacting in solution, the hotprecipitate was collected onto paper by gravity filtration. The productwas rinsed copiously with acetone (Warner Graham Company) and nH₂O toremove residue left behind from the reactants. After drying in air, theoxide was lightly ground and heated in a drying oven at 85° C. for 24hours to evaporate any remaining adsorbed solvent. The control,Fe_(3-x)O₄, was prepared in the same manner by excluding MoCl₅ from thesynthesis.

EXAMPLE 2 Physical Characterization

Structural analysis was conducted using a Bruker AXS D8 ADVANCEdiffractometer equipped with a Cu Kα source (λ=1.5406 Å). Sample powderswere loaded into poly(methyl methacrylate) specimen holders and a singlescan was conducted from 20°-70° on the 2-theta axis with a stepincrement of 0.02°. The integration time for each of the 2500 datapoints acquired was 10 s. The presented XRD patterns (FIG. 1) arebackground subtracted and stripped of the Kα₂ contribution. The XPSmeasurements of the Mo 3d spectral region were conducted on a ThermoScientific K-Alpha spectrometer with a flood gun and a monochromatic AlKα source (1486.7 eV). High-resolution scans were acquired using a passenergy of 20 eV and a dwell time of 100 ms. The step size was fixed at0.15 eV, and the spectra were signal averaged ten times before analysis.XPS data were fit with the 2009 version of Unifit (written by RonaldHesse) using a Shirley background correction (Shirley, Phys. Rev. B, 5,4709-4714 (1972)). Electron micrographs, electron diffraction (ED)patterns, and elemental energy dispersive X-ray (EDX) maps were allacquired on a JEOL 2200FS TEM equipped with a Gatan CCD camera and aNORAN System SIX X-ray Microanalysis System. For analysis, each metaloxide was brushed onto a holey-carbon film support 200-mesh copper grid(SPI Supplies) and multiple images/elemental maps were acquired todetermine the representative compositional and morphological featurespresented herein. The water content was assessed by thermoanalyticalmeasurements. Briefly, 10-15 mg of each metal oxide was placed in anAl₂O₃ crucible and heated in a NETZSCH STA 449 F1 Jupiter TGA interfacedto a QMS 403 Aëolos mass spectrometer. In each analysis the TG-MS wasprogrammed to equilibrate at 50° C. for 10 min and then ramped to 600°C. at 10° min⁻¹ under a controlled stream of O₂ (30 mL min⁻¹). The massspectrometer was used to detect the expulsion of singly charged water(m/z=18) throughout the experiment.

In FIG. 1, the diffraction patterns of Mo-ferrite and the Fe_(3-x)O₄control are shown; both correlate with the cubic γ-Fe₂O₃/Fe₃O₄ latticestructures (Schulz et al., ICDD Grant-in-Aid, North Dakota StateUniversity (1987) PDF# 39-1346; Fjellvåg et al., J. Solid State Chem.124, 52-57 (1996) PDF# 89-0688). The XPS analysis of the Mo-ferritecomposition (FIG. 2) confirms that our base-catalyzed synthesis yields aproduct containing Mo having a chemical state consistent with Mo⁶⁺ inMoO₃ (Bica de Moraes et al., Chem. Mater. 16, 513-520 (2004); Pattersonet al., J. Phys. Chem. 80, 1700-1708 (1976); Fleisch et al., Appl. Surf.Sci. 26, 488-497 (1986)). FIG. 3 shows TEM images of the Mo-ferrite andthe Fe_(3-x)O₄ control samples, with nanoparticles and nano-sizedmorphological features clearly visible. The EDX mapping (FIG. 4)confirms that the Mo ferrite sample is compositionally uniform assynthesized with no elemental segregation between the varyingmorphological features. Thermal oxidation of Mo-ferrite and Fe_(3-x)O₄in hybrid TG-MS experiments (FIG. 5) leads to mass loss during heatingwith water as the major constituent released.

The vacancy concentration of our defect spinel ferrites was approximatedfrom the aforementioned datasets and results not presented here fromX-ray absorption spectroscopy (XAS) experiments and quantitative energydispersive X-ray (EDX) analysis. Based on the XRD data (FIG. 1), weassume that the Mo-ferrite and the Fe₃O₄ materials both adhere strictlyto an AB₂O₄ spinel structure. The oxidation state of Mo in Mo ferritewas determined to be +6 by XPS (FIG. 2), and this was confirmed by X-rayabsorption near-edge spectroscopy (XANES), a specific form of XAS. Theoxidation state of Fe was also approximated by XANES, and for thematerials discussed herein, we calculate Mo-ferrite and Fe_(3-x)O₄ tohave oxidation states of 2.96 and 2.82 respectively. To write molecularformulae with an AB₂O₄ composition, the inclusion of cation vacanciesare necessary to preserve charge balance between the cationic andanionic species and mass balance (i.e., maintain the ratio of threecation sites per four anion sites). Substituting higher valent cationsleads to higher vacancy concentrations (Table 1).

EXAMPLE 3 Electrochemical Charge Storage Studies

The Mo-ferrite and Fe_(3-x)O₄ nanoparticles discussed above served asactive electrode materials for Li⁺ and Mg²⁺ insertion/extractionexperiments. The oxide under investigation was mixed with acetyleneblack (Alfa Aesar) and Kynar® HSV 900 polyvinylidene fluoride (PVDF)resin in an 84:10:6 weight ratio using N-methyl-2-pyrrolidone (Aldrich)solvent. The resulting paste was coated onto aluminum (Strem) flags anddried overnight at 50° C. under a stream of flowing N₂ gas. Thefollowing day, the coated substrates were heated to 100° C. or 150° C.in vacuo for three hours to solidify the composite and ensure theremoval of adsorbed water. Finished composite electrodes werecharacterized by cyclic voltammetry and galvanostatic charge-dischargemeasurements in a glovebox filled with high purity Ar. Using aconventional three-electrode half-cell arrangement, the compositeelectrode was immersed in a 1 M LiClO₄/propylene carbonate solution withtwo strips of Li ribbon acting as the reference and auxiliary electrodesfor Li⁺ charge-storage studies. Prior to cyclic voltammetrymeasurements, the working electrode was preconditioned at 4.1 V vs Lifor 10 minutes; immediately afterwards the potential was scanned to 3.0V, 2.5 V, or 2.0 V (vs. Li) at 500 μV s⁻¹ as indicated. Galvanostaticcharge-discharge cycles were conducted between 4.1 V and 2.0 V (vs Li)using an applied current of 10 mA g⁻¹. (Note the current is normalizedto the mass of metal oxide present). For the Mg²⁺ charge storagestudies, the composite electrode was immersed in 1 M Mg(ClO₄)₂/propylenecarbonate with Ag wire acting as a reference electrode and steel actingas an auxiliary electrode. Galvanostatic discharge cycles using theMo-ferrite composite electrode were conducted between 2.55 V and 0.15 Vvs Mg using an applied current of 20 mA g⁻¹. (Again, the current isnormalized to the mass of metal oxide present).

FIG. 6 compares cyclic voltammograms of Fe_(3-x)O₄ composite electrodesto Mo-ferrite composite electrodes immersed in 1 M LiClO₄/propylenecarbonate, and as the data clearly show, the magnitude of the normalizedcurrent is significantly larger for voltammograms acquired using theMo-ferrite composite electrode. The Li⁺ charge-storage capacity wasdetermined directly from galvanostatic charge-discharge curves. From thedata in FIG. 7, Mo-ferrite achieves Li⁺ capacities 4-5 times that ofFe_(3-x)O₄ over a moderate potential window (4.1-2.0 V vs Li). Moreover,the Coulombic efficiency of Mo-ferrite is >94% for all three cyclessuggesting that structural degradation is gradual and thecation-deficient ferrite reversibly intercalates Li⁺ over many cycles.Mo-ferrite can also intercalate Mg²⁺ (FIG. 8) with decent reversibilityover the first three cycles.

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A composition comprising: an oxide comprisingions of a first metal and ions of a second metal; wherein the secondmetal ions have a higher oxidation state than the first metal ions;wherein the presence of the second metal ion increases the number ofmetal cation vacancies relative to an absence of the second metal ion;and wherein the metal oxide is proton stabilized.
 2. The composition ofclaim 1; wherein the first metal ion is Fe³⁺; and wherein the secondmetal ion is Mo⁶⁺ or V⁵⁺.
 3. The composition of claim 1, wherein theoxide comprises γ-Fe₂O₃ and Mo⁶⁺ or V⁵⁺.
 4. A method comprising:dissolving salts of a first metal ion and a second metal ion in water toform a solution; heating the solution to a temperature of about 80-90°C.; and adding a base to the solution to precipitate nanoparticles of anoxide of the first metal ion and the second metal ion; wherein thesecond metal ions have a higher oxidation state than the first metalions; and wherein the presence of the second metal ion increases thenumber of metal cation vacancies relative to an absence of the secondmetal ion.